The teachings of the above-referenced U.S. Pat. No. 5,482,611 to Helmer et al. have been found to be very useful in providing a physical vapor deposition source (frequently referred to as a "sputtering" source) which exhibits an improved directionality and degree of ionization of the target metal vapor while achieving a commercially acceptable deposition rate. The Helmer patent essentially teaches a magnetron sputter source containing a hollow cathode target (HCT).
This structure is generally illustrated in FIG. 1, which shows a magnetron sputter source containing a "hollow" target 2. The cup-shaped cavity in target 2 is surrounded by walls 5-5'. The permanent magnet 4-4' provides magnetic field lines which are generally parallel to the surfaces of walls 5-5' inside the cavity and which form a magnetic null in the region designated 1 at the opening of the cavity.
In operation, the pressure in the reaction chamber is reduced to, for example 10.sup.-5 torr or less. A small quantity of an inert gas such as argon is introduced into the reaction chamber, increasing the pressure to the range of 1-5 millitorr, for example. When a plasma discharge is created, in a known manner, by applying a high negative voltage to the target 2, this arrangement produces a doughnut-shaped plasma 7 (shown by the dashed lines) along the walls 5-5'.
The width W and depth of the target cavity are on the same order dimensionally. This provides a high probability that the neutral target atoms sputtered from the walls 5-5' will be either ionized by the highly intense plasma or redeposited on the opposite wall and then resputtered back towards the wall from which they originated. For example, it has been computed that 54.8% of the sputtered atoms are ionized, as compared with 2% of the atoms in prior art magnetron sputter sources. (The ionization percentage is sometimes referred to as the "plasma intensity".) The magnetic null region 1 at the upper edge 6 of the region 3 traps and retains ions and electrons inside the cavity except for those particles which enter the null region 1 with an axial velocity and very little radial velocity. Thus, ions and electrons which have primarily an axial velocity are able to leave the cavity along the axis at the upper edge of the region 3. Other particles are "reflected" back into and contained within the cavity.
Several advantages derive from using the magnetic "mirror" to extract the plasma. Once the electrons leave the discharge region, the null mirror isolates the electrons in the plasma and prevents the beam from coupling with the electrons inside the cavity. Therefore, the exiting plasma beam can be manipulated or biased without affecting the discharge characteristics in the cathode. By isolating the extracted plasma from the discharge, the confining null-field magnetron cathode is far more flexible than most other plasma sources. Another advantage is that the transverse velocity of the plasma beam is very small. This allows the plasma beam to be steered, focused or expanded using small magnetic or electric fields.
One problem that can occur, however, is that the erosion of the HCM target is non-uniform. In particular, the center of the closed end and comers of the target may show a net deposition. It has been determined that this is due to the shape of the magnetic field lines in these areas.
The above-referenced application Ser. No. 09/073,358 teaches that the erosion profile of an HCM target can be controlled using pole plates, tapered sidewall magnetic fields, a rotating magnet array placed adjacent the closed end of the HCM target, or a combination thereof. The rotating magnet array, in particular, can enhance the erosion of the closed end of the target and opens the possibility of achieving full-face erosion of the entire HCM target. However, problems may be created by the interaction of the main magnetic field with the magnetic field produced by the rotating magnet array. This is predominantly due to the fact that the rotating magnet array tends to support a separate plasma discharge on the target surface adjacent to it which in turn competes with the main HCM plasma discharge.
In a configuration where the magnetic field produced by the rotating magnet array is aligned with, or "aids", the main magnetic field, erosion is increased at the outer diameter of the closed end but reduced near the center of the closed end. Where the magnetic field produced by the rotating magnet array is aligned against, or "bucks", the main magnetic field, erosion is reduced at the outer diameter of the closed end but increased near the center of the closed end.
Ideally, the erosion profile should be uniform to permit maximum utilization of the target and to minimize the number of times that the spent target must be replaced. It is also important to achieve a uniform erosion of the entire target to prevent flaking of back-scattered deposits of the target material. Such flakes can contaminate the substrate that is being processed.
Thus there is a need for even more effective techniques of controlling the target erosion profile in an HCM sputter source.